Update class inheritance to support multi-parent schemas (biothings/discovery-app#330)

[NikkiBytes]

Enhance parent validation merging with support for multi-inheritance and list merging

This update improves the recursive parent validation logic in validator.py:

• Supports multiple rdfs:subClassOf parents (previously only a single parent was handled).
• Introduces visited tracking to prevent cyclic recursion or duplicate merging.
• Updates merge() to combine lists (e.g., merging "required" fields across parents), instead of skipping or overwriting them.
• Ensures more complete and accurate schema validation when multiple parents define validation logic.

These changes enable the system to correctly handle complex ontologies with multi-inheritance and overlapping validation fields.

[NikkiBytes]

Notes on Validator Enhancements (biothings_schema/validator.py)

• New _norm_id method

  • Strips whitespace.
  • Expands CURIEs using known prefixes.
  • Normalizes schema.org variants (http → https and www removal).
  • Ensures consistent trailing slash removal.

• New _merge_lists method

  • Merges lists with de-duplication.
  • Handles complex list items (dict/list) using JSON serialization for uniqueness.

• Enhanced merge method

  • Supports merging nested dictionaries.
  • Uses _merge_lists for merging list values.
  • Performs deep copies to avoid shared mutable references.

• Refactored merge_recursive_parents

  • Now handles multiple inheritance (rdfs:subClassOf as list or dict).
  • Tracks visited parent IDs to prevent infinite loops.
  • Normalizes IDs before lookup to ensure http/https and CURIE consistency.
  • Recurses cleanly up the class hierarchy, merging $validation fields from all ancestors.